Various limitations exist for accessing an apparatus (a terminal apparatus) connected to a LAN, which is connected to the Internet via a gateway apparatus, from an apparatus (a server apparatus) connected to the Internet.
For example, here is a technique, in which when, a terminal apparatus does not have a global IP (Internet Protocol) address but only has a local IP address, a gateway apparatus (a router) converts the IP address or a port number using a NAT (Network Address Translation) function or a NAPT (Network Address Port Translation) function.
When the IP address or the port number of the terminal apparatus is converted using the NAT function or the NAPT function, a server apparatus knows the IP address or the port number of the terminal apparatus by a request from the terminal apparatus. Accordingly, it is difficult to actively access the terminal apparatus from the server apparatus side.
To solve the above problem, there are techniques, for example, Patent Document 1.
Further, from the viewpoint of security, there is a technique that a gateway apparatus limits accesses from a server apparatus to a terminal apparatus (so-called firewall).
Among various firewall techniques, a method is popularly used, in which an access from the terminal apparatus to the server apparatus and its response from the server apparatus are allowed, and other accesses from the server apparatus to the terminal apparatus are prohibited.
Patent Document 1: JP2004-120547